In circuit breakers, the state of the switch is often determined for diagnosis purposes. Important states of a switch are the low-load situation, the normal situation or the overload situation. These situations can be determined by measuring the current through the switch. If the supply line is interrupted, the low-load situation or open-load situation obtains. If the low-load situation and the overload situation are determined by measuring currents in the switch, it is necessary for the current thresholds for the low-load situation and the overload situation to be as far apart as possible so that the states can be explicitly determined.
Circuit breakers can be used as high-side switches, low-side switches or motor bridge drivers. In a motor bridge with a rated current of 1 A, currents in the switch above 1.5 A can be interpreted as an overload and currents below 10 mA can be interrupted as a low load. A large ratio for the rated current to the rated low-load current or for the rated current to the rated overload current often requires not inconsiderable circuit complexity. A low-load detector uses suitable measures to ascertain the current through the load. If there is an interruption in the load path or if the load resistance is too high, a diagnosis apparatus should identify this state. When the low-load state has been identified, a diagnosis circuit outputs a low-load situation signal. In the case of high-side switches using bipolar technology, for example in an NPN-PNP Darlington arrangement, ascertaining the low-load situation requires a high level of circuit complexity.
A known solution for diagnosing the low-load situation in a bipolar high-side switch is a resistor in series with the high-side switch. The voltage produced as a result of the current through this resistor is compared with a reference voltage using a comparator, and if it is below the reference voltage then this indicates the low-load situation. The additional voltage drop across the resistor is troublesome, since it increases the resistance of the high-side switch to a corresponding degree.
To sense the low-load situation, it is often necessary to sense small currents via the high-side switch. Sensing small currents results in the need for relatively large resistors to be used, since the voltage drop across the resistor needs to be large enough for disturbing influences on the comparator, such as the offset voltage in the comparator, not to be dominant. Since the comparator is part of the high-side switch, the comparator experiences the same voltage swing as the high-side switch. The high voltage swing means that the comparator needs to have a high level of common-mode rejection. The high common-mode rejection means a high level of circuit complexity.
Another known option is a current mirror circuit. Current mirror circuits are technically complex and sensitive towards temperature influences.
Another known option is a low-side switch having two NMOS transistors of different size, the smaller NMOS transistor being provided with a source resistor. If the current through the low-side switch is below a predetermined threshold, the open-load situation is identified. The split of the low-side switch allows small currents to be measured without significantly impairing the switched-on resistance of the low-side switch.
Another known option is a high-side switch having two NMOS transistors of different size, the small NMOS transistor carrying a reference current. The reference current value is used to define the open-load situation for the high-side switch. The reference current value is set taking account of the area ratios of the NMOS transistors of different size.
Another known option is a PNP transistor as a load transistor having a measurement zone for sensing the saturation current of the PNP transistor and having an evaluation apparatus.